During an initial access, a reconnection, or a handover in which a mobile station (hereinafter also referred to as a UE) starts a communication with a base station (hereinafter also referred to as an eNB), the mobile station (UE) performs a negotiation by sending a call request signal to the base station (eNB). This process will now be described by taking LTE (Long Term Evolution) as an example, which is one of the next-generation mobile phone standards that are being standardized.
FIG. 1 is a diagram illustrating an example of an initial access sequence of the LTE. In FIG. 1, when a UE 1 makes an initial access, a preamble number is selected, and a call request Messsage1 is sent to an eNB (step S1). The eNB detects the power of the Message1, detects a timing error of the eNB from a reference timing, and sends a call request response and an uplink communication enable signal, as Message2, to the UE 1 (step S2). The call request response includes a timing control signal (TA: Time Advance) that controls a transmission timing of the UE 1 so that the signal from the UE 1 is received by the eNB at the reference timing.
FIG. 2 is a diagram illustrating an example of a data format of the Message2. The Message2 includes a preamble number (Preamble No.) selected by the UE 1, the timing control signal (TA: Time Advance), information UL_grant granting allocation of an uplink radio resource to be used by the UE 1, and a user ID C-RNTI (Cell Radio Network Temporary Identifier) allocated for each cell. In the case of the LTE, the band (RB number) used by the UE 1, the data number (TBS), the modulation type, and the like are notified by the information UL_grant. One radio resource (Time Advance, UL_grant, and C-RNTI) is allocated from the eNB to the UE 1 by the Message2.
Returning now to the description of FIG. 1, when the UE 1 receives the Message2, the UE 1 judges whether the Message2 is addressed thereto based on the preamble number thereof. If the Message2 is addressed to the UE 1, the UE 1 shifts the transmission timing, and sends to the eNB a connection (RRC) request Message3 that includes information unique to the UE 1, using a notified radio resource (step S3). The eNB demodulates the received Message3, and if an error detection using a CRC (Cyclic Redundancy Check) succeeds, the eNB sends a connection response Message4 to the UE 1 (step S4). The negotiation is completed when the UE 1 receives the Message4.
In the case illustrated in FIG. 1, if a round-trip time is 4 msec, for example, the negotiation time required for the negotiation to be completed is approximately 12 msec.
Methods of reducing the negotiation time of a link establishing negotiation between the eNB and the UE are proposed in Japanese Laid-Open Patent Publications No. 2007-134946 and No. 2008-148362, for example.
When a plurality of UEs send the call request Message1 having the same preamble number, the negotiation time becomes relatively long. Particularly in the case of the LTE, only 64 Preamble Nos. exist, and the possibility of the plurality of UEs making the transmission using the same preamble number is relatively high.
FIG. 3 is a diagram for explaining a case in which UEs 1 and 2 send the call request Message1 having the same preamble number. In FIG. 3, when the UEs 1 and 2 send the Message1 having the same preamble number (step S11), the call requests Message1 from the UEs 1 and 2 appear as multipath signals to the eNB. The eNB detects each power of the Message1, calculates the Time Advance from the Message1, and sends a call request response and an uplink communication enable signal, as Message2, to the UEs 1 and 2 (step S12).
Each of the UEs 1 and 2 sends a connection (RRC) request Message3, including the information unique to the UE, to the eNB based on the information of the Message2 that is received (step S13). The eNB demodulates each of the Message3 that is received, but the demodulation will be unsuccessful because the information unique to the UE 1 and the information unique to the UE 2 differ. For this reason, the eNB does not send a connection response Message4 (step S14). FIG. 3 illustrates each signal that is not sent by dotted arrows.
When the UEs 1 and 2 do not receive the Message4, the UEs 1 and 2 resends the Message3 a preset number of times, which may be 4 times, for example (steps S15 and S17).
However, because the UEs 1 and 2 sends the Message3 using the same radio resource, the eNB is unable to demodulate the Message3, and thus, the eNB does not send the Message4 (steps S16 and S18).
Because the UEs 1 and 2 after resending the Message3 the preset number of times are unable to receive the Message4, the UEs 1 and 2 detect a collision, and send the Message1 in order to start the negotiation again (step S19). For this reason, a minimum negotiation time is 56 msec (=44 msec+12 msec) for the above example, and there was a problem in that a relatively long negotiation time is required.